1. Field of the Invention
This invention relates to a fixing device for performing a fixing operation wherein feed members, such as rollers, belts and the like, grasp and feed a supporting material on which an unfixed toner image has been deposited.
2. Description of the Related Art
After forming a toner image on a recording material, such as paper or the like, by a method such as the Carlson process in electrophotographic apparatus, the toner image must be fixed as a permanent image. Various methods have been proposed as fixing methods for use in electrophotographic and other image forming apparatus. Recently, from the viewpoint of thermal efficiency and economy, heat-roller fixing devices are frequently used in which a toner image is fused and fixed by grasping and feeding a recording material with a pair of rollers, a fixing roller and a pressing roller, at least one of which has a heating source.
FIG. 5 shows a heat-roller fixing device which comprises a fixing roller 1 having a heating source 8, such as a halogen lamp or the like, and a pressing roller 7 having an elastic layer made of silicone rubber or the like. The pressing roller 7 is in pressure contact with the fixing roller 1. As described above, while the two rollers rotate and a recording material P, such as paper or the like, passes between the rollers, a toner image T fuses by heat and pressure, and cools after leaving the pair of rollers to form a fixed image.
There is also shown a frame 10 for the fixing device.
In such a fixing device, the phenomenon of offset easily occurs because sticking materials, such as fused toner and the like, adhere to the fixing roller when the surface of the fixing roller directly contacts the toner image. When offset occurs, the toner adhering to the fixing roller as a result of offset is fed on the roller, and adheres again to the recording material P, causing contamination of the image. Hence, it is necessary to prevent the offset phenomenon in such a fixing device.
As one method for reducing or preventing the offset phenomenon, there is a method in which potential on the surface of the fixing roller is controlled. In this method, the mechanism of the occurrence of offset is investigated, and from the viewpoint that offset occurs due to an electrostatic force at the fixing portion, the electrostatic force is controlled. Offset is a phenomenon which occurs due to the transfer of toner from the recording material P to the fixing roller 1. The forces causing the transfer and to be considered are electrostatic forces on the toner in the direction of the fixing roller and adhesion force due to the tackiness of the toner. That is, in a conventional hot-roller fixing device as shown in FIG. 5, in general, the fixing roller 1 is negatively charged at ten volts to several hundreds volts, the pressing roller is charged at a higher potential of several hundred volts to several tens of thousands of volts, and the recording material P has positive electric charges. As a result of the above-described potential relationship, an electrostatic force is exerted between the toner and the fixing roller.
For example, when fixing positively-charged toner, if the triboelectrification potential between the fixing roller and the recording material is high and the surface potential of the pressing roller is low, a strong electrostatic attractive force is exerted between the fixing roller and the toner, and the toner adheres to the fixing roller.
When fixing negatively-charged toner, if the potential of the fixing roller is low and the negative potential of the pressing roller is high, a strong electrostatic repulsive force is exerted between the pressing roller and the toner, and the toner adheres to the fixing roller.
Methods for preventing or effectively utilizing the above-described electrostatic forces have been considered.
In Japanese Laid-Open Patent Application No. 55,374/1980, a method has been disclosed in which, by disposing corona discharge units facing outer circumferential surface of rollers, or by making contact biasing rollers, charge-removing cloths or the like with outer circumferential surfaces of rollers, electric charges on either a fixing roller or a pressing roller are removed to reduce offset.
However, if the potential of a roller is controlled from the outer circumferential surface of the roller, the potential is apt to be influenced by changes in environment. In addition, the configuration of the device becomes large.
In consideration of the above-described disadvantages, in Japanese Patent Public Disclosure (Kokai) Publication No. 55-96970 (1980), a biasing voltage is applied to a core bar of a roller to prevent offset.
In U.S. patent application Ser. No. 446,426, filed Dec. 5, 1989, and assigned to the assignee of this application, the core bar is grounded via a diode.
Although the above-described application of a biasing voltage to the core bar is effective for the prevention of offset in some cases, offset has not been sufficiently prevented in cases where the triboelectrification potential of the surface of the roller is high.
For example, in Japanese Patent Public Disclosure (Kokai) Publication Nos. 54-2137 (1979), 57-150869 (1982) and 58-209769 (1983), there are descriptions of reducing resistance values of fixing and pressing rollers to reduce triboelectrification of the rollers.
These approaches are, however, for reducing electrostatic influences, and there has been no description about the prevention of offset utilizing an electrostatic force.
If the surface of the above-described roller having a low resistance value is microscopically observed, resin or rubber and conductive powder are merely mixed. Hence, the above-described approaches still have the problem that toner particles contacting resin portions receive electrostatic attractive forces, causing offset.